Autolink bare URLs in lesson comments

Comments render CommonMark, which leaves a pasted https://… as dead text —
students share links constantly and nobody writes [text](url) by hand. Add
remark-gfm so plain URLs, www hosts and emails become links (and tables and
strikethrough come along), plus table styling so a wide table scrolls inside
itself instead of stretching the page.

Sanitizing is unchanged: javascript: is neither linked nor autolinked, raw
HTML is still dropped, and third-party images are still rendered as plain
links by the img component.

src/app/globals.css

@@ -243,6 +243,24 @@
 }
 
 /* Markdown-разметка в комментариях под уроками (react-markdown) */
+/* Таблицы GFM: широкая таблица прокручивается внутри себя, а не растягивает страницу */
+.comment-md table {
+  display: block;
+  width: max-content;
+  max-width: 100%;
+  overflow-x: auto;
+  border-collapse: collapse;
+  margin: 0.5rem 0;
+  font-size: 0.95em;
+}
+.comment-md th,
+.comment-md td {
+  border: 1px solid var(--border);
+  padding: 0.35rem 0.6rem;
+  text-align: left;
+}
+.comment-md th { font-weight: 700; background: var(--color-surface); }
+.comment-md del { opacity: 0.6; }
 .comment-md img {
   display: block;
   max-width: 100%;

src/components/student/lesson-comments.tsx

@@ -2,6 +2,7 @@
 
 import { useRef, useState, useTransition, type ReactNode } from "react";
 import ReactMarkdown from "react-markdown";
+import remarkGfm from "remark-gfm";
 import rehypeSanitize, { defaultSchema } from "rehype-sanitize";
 import { addComment, deleteComment, editComment } from "@/lib/actions/student-actions";
 
@@ -63,6 +64,9 @@ function CommentBody({ text, imageHost }: { text: string; imageHost: string }) {
   return (
     <div className="comment-md text-sm leading-relaxed break-words">
       <ReactMarkdown
+        // GFM: голый адрес сам становится ссылкой. Без него студент пишет
+        // https://… и получает мёртвый текст — разметку никто не проставляет.
+        remarkPlugins={[remarkGfm]}
         rehypePlugins={[[rehypeSanitize, sanitizeSchema]]}
         components={{
           // eslint-disable-next-line @typescript-eslint/no-unused-vars -- отбрасываем служебный node от react-markdown, чтобы он не протекал в DOM